Radio transmission system



July 23, 1929.

IIII

Patented `luly 23, 1929.

UNITED STATES PATENT OFFICE.

LAWRENCE C. F. HORLE, OF NEWARK, NEW JERSEY, ASSIGNOR T0 FEDERAL TELE-PHONE MANUFACTURING CORPORATION, OF BUFFALO, NEW YORK, A CORPORA- TIONOF NEW YORK.

RADIO TRANSMISSION SYSTEM.

Application filed August 6, 1924. Serial No. 730,485.

This invention relates to radio transmission systems and moreparticularly to a transmission system in which a number of radiobroadcasting stations are arranged for simultaneously radiating speechsignals or other signals originating in a single master control stationconnected by line wires to the broadcasting stations.

It has been proposed heretofore to broadcast speech or otherradio'signals, originating at a central master station, simultaneouslyfrom widely separated stations by transmitting to all of thebroadcasting stations the speech or other signal wave over line wiresfrom the master station, each station of the group of broadcastingstations generating its own individual carrier wave, independently ofthe master station, and the carrier wave of any given station having awave length different from that of others of the group. In such systemsthe energy received over the line wires from the master station hascarried only the audio* frequency speech or signal wave which has thenbeen impressed upon the various carrier waves of the broadcastingstations as a modulating component of these carrier waves.

It is frequently desirable, however, that the speech or other signalsthus originating in a master station and transmitted over line wires toa group of radio broadcasting stations be radiated from all the membersof the group of broadcasting stations on carriers of the same, insteadof different, wave length for the individual stations of the group. Butin order that such broadcasting of the same speech or other signal onthe same carrier wave length from a number of stations simultaneouslymay be accomplished satisfactorily, it is imperative that the wavelengths of the carriers radiated from the individual members of thebroadcasting group be maintained indentically the same. If these carrierwave lengths' from the respective stations are permitted to differ,reception of the broadcast radio signals by a receiver so situated as.to be within the broadcasting range of more than one of the group ofstat-ions becomes ditlicult or impossible, because of bea-ts due to thedifference in wave length of the carriers radiated from the variousstations.

To obviate this dihculty in reception of the broadcast modulated signalWaves from the group of stations, it is proposed to provlde anarrangement such that the energy transmitted over line wires from themaster station may not only act to modulate the carrier waves of theseveral broadcasting stations with a speech or other audible componentbut may also act as a control means so related to the transmitting meansat the several stations of the broadcasting group that the carrier wavesradiated from the said several stations are maintained at the samefrequency each to each. This control of broadcasting frequency of the`carrier waves radiated from a number of interconnected stations bymeans of energy transmitted over line wires from a distant mastercontrol station cannot, however, be attained by the direct transmission,over the wires, of a carrier wave having a frequency equal to that ofthe radio waves which are to be radiated as carriers from thebroadcasting stations. As is well known, the transmission over linewires of waves of the order of radio frequency, that is, of theenormously high frequency usual in radio tele-l phony broadcasting. andfrom a control station to a distant broadcasting station, isimpracticable, due to causes well recognized in the art, such asattenuation and distortion which increase with the frequency. Further.it is obvious that the problem of main taining the frequency of thecarrier waves radiated from the several stations equal to each is alsonotto be solved by attempting to change the frequency of thesebroadcasting carrier waves to meet the conditions imposed bytransmission over long line wires, since this procedure means areduction of the broadcasting frequency to the frequency suitable toline wire transmission, and consequently to a point far below thatrequired in practice for radio telephony. The solution of the problem,therefore, of so controlling, over line wires, the carrier waves of agroup of broadcasting stations from a distant master control station asto hold the frequencies of these broadcasting carrier waves identicallythe same, calls for means and methods which permit the full use of suchline wire carrier wave frequencies, say 10000 to 20000 cycles, as havebeen found efficient in line wire transmission, and at the same time itcalls for means and CIK M retained all the advantages inherent in theusual method of simultaneous broadcasting on different wave lengths fromseveral stations of the speech or other signals transmitted overlinewires from a master station, and in which, further, may be fullyVrealized the advantages inherent in the method of broadcasting,simultaneously from'such groups of stations, on the same wave length.

`Inaceordance with the invention, energy in the form of a line wirecarrier wave, of a frequency best suited to line wire transmission, issent over line wiresto the several stations of the broadcasting group.This line wire carrier wave is impressed upon suitable means at each ofthe stations, by which the wave is amplified, and changed in frequencyto the high value required for broadcasting, the frequency of thebroadcasting wave being, however, maintained the same for all members ofthe group of broadcasting stations. At some point in the carrier wavepath through the complete system of master control station andbroadcasting station, the carrier wave is modulated by a speech or otheraudio frequency signal wave provided through the action. of suitablemea-ns in the master control station. The amplified and modulated highfrequency waves are finally rediated in the usual manner simultaneouslyfrom the several stations of the broadcasting group.

The invention will be best understood as to organization and method ofoperation and as to its other objects and advantages by reference to thefollowing detailed description 'and to the drawings, which illustratetwo embodiments of the invention.

Figure 1 represents schematically an embodiment of the invention inwhich a speech or other audio frequencysignal Wave and a line wireunmodulated carrier wave. are separately transmit-ted from controlstation to broadcasting station.

Figure 2 represents schematically an embodiment in which a modulatedline wire carrier wave is transmitted from control station tobroadcasting station.

Figure 3 is a diagrammatic illustration more in detail of certainapparatus utilized in lthe Vembodiment shown in Fig. 2.

The saine part wherever it appears in the various figures will belrepresented by the same reference character.

Referring to Fig. 1, a master control station 1 comprises apparatusrepresented schematically by 2 for generating speech or other signalwaves and also comprises apparatus represented by 3 for generating aline wire carrier wave. Broadcasting stations as 4 and 5 are-located ata distance from control station 1. Each of the broadcasting stationscomprises the following apparatus represented schematically in thefigure: Speech or other signal supply waveamplifier 0; line arrier wavedistortion amplifier 7; wave filter 8; radio frequency amplifier 9;modulator and radio frequency power amplifier l0; transmitting antenna11. Line wires 12 connect the line carrier wave generating apparatus 3of the master control station 1 to the line carrier wave distortionamplifier 7 of a broadcasting station, as station 4. Line 'wires 13connect the speech or other signal wave generating apparatus 2, of themaster control station 1, to the signal wave amplifier 6 of abroadcasting station. In operation, a carrier wave of a frequency aboveandibility but of the low value required in efficient line wiretransmission, and of the order, for example of 10000 to 20000 cycles, isgenerated by any suitable means in wave generating apparatus 3 of themaster control station. The carrier wave is transmitted in a usualmanner over line wires 12 to distant broadcasting stations Lich as 4Land 5. At a given broadcasting station the wave, of line wire carrierfrequency, is distorted by any well known and suitable means as by adistortion amplifier 7 to produce a desired harmonic frequency of thesaid incoming line wire carrier wave. This desired harmonic is selectedby wave filter 8 through which the wave from the distortion amplifier ispassed. rfhe frequency of the line carrier wave transmitted over theline wires 12 and used as a basis for the harmonic is so chosen that thesaid harmonic is of the frequency desired for the radio frequency wavewhich is to be broadcasted from the stat-ion. The radio frequencycarrier wave resulting from the selective action of wave filter 8 issuitably amplified as by R. F. amplifier 9. The mochilation of thisradio frequency carrier wave from amplifier 9 is effected by means of aspeech or other audible signal wave transmitted from audible signal wavegenerator 2, over line wires 13, through audible signal amplifier 6 tothe modulator and R. F. power amplifier 10. In this modulating andamplifying apparatus 10 the amplified audible signal wave is impressedupon theamplified R. F. carrier wave, and the resulting modulated R. F.wave'is then broadcasted in the usual manner from antenna 11.

Referring to Fig. 2, a master control station 14 comprises apparatusrepresented by 15 for generating speech or other signal waves in anysuitable manner as by the same means as employed in the apparatus 2 ofFig. 1. The control station 14 also comprises speech or ot-her audiblesignal amplifier 16, and line carrier wave generator and modulator 17.Broadcasting stations, as 18 and 19, are located at a distance fromcontrol station 14. Each of these broadcasting stations comprises thefollowing apparatus: modulated line carrier wave distortion amplifier20; wave filter 21, radio frequency power amplifier 22; transmittingantenna 23. Line wires 24 connect the line carrier wave generator andmodulator 17, of the master control station 14, to the distortionamplifier 2O of the broadcasting station. In the operation of the systemas arranged in Fig. 2, a carrier wave of a frequency above audibilitybut of the comparatively low frequency required for eflicient` line wiretransmission is generated at the master control station by any suitablemeans such as by apparatus represented at 17 A speech or other audiblesignal wave generated by any usual or known means as -by audible signalWave generator 15, and suitably amplified as by signal wave amplifier1G, is impressed at the master control station upon the'line wirecarrier wave, as by means of the apparatus represented by the linecarrier wave generator and modulator 17. The modulated line carrier waveis then transmitted, essentially as in the case of the line carrier waveof Fig. 1, over line Wires 24 to distant broadcasting stations such as18 and 19. At each of the broadcasting stations the modulated linecarrier wave is distorted, as by distortion amplifier 20, to produce aharmonic of the incoming Wave, and the desired harmonic is selected asby wave filter 21. As in the case of the line carrier wave of Fig. 1,the frequency of the line vcarrier Wave used as a basis for the harmonicis such that the said harmonic is of the frequency desired forbroadcasting. The modulated radio frequency wave from filter 21 issuitably amplified as by R. F. amplifier 22 and is then broadcasted fromantenna 23.

The preferred embodiment of the invention which is outlinedschematically in Fig. 2 is illustrated more in detail in thediagrammatic representation of Fig. 3, which shows apparatus which maybe used in a master control station and in a member of a group ofbroadcasting stations, as arranged in accordance with the invention.Referring to Fig. 3, a master control station 25 is shown, correspondingto master control station 14 of Fig. 2. A source of speech or otheraudio frequency modulating current supply 26, comprising microphone 27and a current source 28 is coupled to amplifying means such as vacuumtube amplifier 29. The output of the amplifier 29 is passed throughsuitable transformer coupling means to a carrier wave generating andmodulating means, as tube 30. The circuits of this tube are arranged ina usual manner for use of the tube as an oscillator for the generationof the carrier wave required for transmission over the line wires andalso as a modulator of the carrier wave by the modulating speech orother audio frequency current impressed on the input circuit of thetube. The modulated linel carrier wave produced in tube 30 istransmitted over line wires, as 31 and 32, to the members of a group ofbroadcast-ing stations, one of which, 33, is represented in the figure.The carrier wave over line 31 is impressed upon the input circuit of atube 34, the circuits of which are so arranged in a usual manner as byoverloading the tube that the tube acts as a distortion amplifier of theline wire wave. The output circuit of tubev 34 is impressed upon anyusual filter means represented by 35, by which a harmonic of properfrequency is selected. As explained in connection with Fig. 2, theharmonic selected is of the frequency required for broadcasting. Theradio frequency modulated wave from the filter means 35 is'amplified asby power amplifier tube 36 and finally radiated from antenna 37.

The schematic Figures 1 and 2 and the more detailed diagrammatic Fig. 3arel representative of embodiments of the invention in that they showthe several conversions that. go on in the systems. It is to beunderstood, however, that in practice various additions to the apparatusillust-rated may be made, such for example as many stages ofamplification where necessary. It is to be further understood that suchadditions, and

likewise many changes in the apparatus described and illustrated may bemade without departing from the scope of the invention as defined in theclaims.

What is claimed is:

1. A radio transmission system adapted to supply broadcast programs to aplurality of radio receivers and comprising a master control station anda group of radio transmitt-ing stations, a control frequency channelcomprising line wires linking the control station and the transmittingstations, said transmitting stations beng spaced from each other by suchdistances that each individual member of the group may constitute aseparate distributingcenter for radio broadcasting, whereby a givenreceiver may be Within the signal audibility range of one only or of aplurality of said group, means at the cont-rol station for generating acarrier wave for transmission over the control channel and having thecomparatively low frequency of line wire carrier waves, means at thecontrol station for generating a signal wave and means for modulating atthe control station said carrier wave by the signal wave, means at eachof the transmitting stations for increasing by the same amount thefrequency of the modulated carrier Wave to a comparatively high radiocarrier frequency, and means at each transmitting station for radiatingthe modulated carrier Wave of increased frequency.

2. A radio transmission system adapted to supply broadcast programs to aplurality of radio receivers and comprising a master control station anda group of radio transmitting stations, a control frequency channelcomprising line wires linking the control station and the transmittingstations, said transmitting stations being spaced from each other bysuch distances that each individual member of the group may constitute aseparate distributing center for radio broadcasting whereby a givenreceiver may be Within the signal audibility range of one only or aplurality of said group, means at the Control station for generating acarrier vvave for transmission over the control channel and having thecomparatively low frequency of line Wire carrier Waves, means at thecontrol station for generating a signal Wave and means for modulating atthe control station said carrier Wave by said signal Wave,

distortion means at each of the transmitting stations for producing aharmonic of the modulated carrier Wave generated at the control station,means at each of the transmitting stations for increasing the frequencyof said Wave by the same amount by filtering out adesired harmonic ofthe Wave, and means at each transmitting station for radiating themodulated carrier Wave of increased frequency. Y

3. In combination, a plurality of radio transmitting stations, a controlstation, a control frequency channel linking said control station andsaid radio transmit-ting stations, means for generating a controlfrequency associated with. said control frequency channel, means at thecontrol station for modulating the control frequency by potentialsvarying at audio frequency, and means at each transmitting station forderiving a band of modulated radiation frequencies from said modulatedcontrol frequency.

In Witness whereof, I hereunto subscribe my name.

LAWRENCE o. F. HoRLE.

